An operational amplifier is a widely-used component of a circuitry. The operational amplifier has a non-inverting input terminal (+), an inverting input terminal (−), and an output terminal. The operational amplifier may be constituted as a unity-gain buffer, also known as a unity-gain follower. FIG. 1A is a schematic circuit diagram of a typical unity-gain buffer. FIG. 1B is a plot illustrating associated signal of the unity-gain buffer of FIG. 1A. Please refer to FIGS. 1A and 1B. In the unity-gain buffer 10, the inverting input terminal (−) of the operational amplifier OP is connected with the output terminal of the operational amplifier OP. The non-inverting input terminal (+) of the operational amplifier OP is served as the input terminal IN of the unity-gain buffer 10 for receiving an input signal. The output terminal of the operational amplifier OP is served as the output terminal OUT of the unity-gain buffer 10.
The unity gain buffer 10 has a voltage gain of unity. That is, the output voltage at the output terminal OUT is equal to the input voltage at the input terminal IN. As shown in FIG. 1B, if a step function is received by the input terminal IN, the input voltage rises up from 0V to Vc at the time spot t0. Since the output voltage at the output terminal OUT fails to instantly respond to the change of the input terminal IN, the output voltage at the output terminal OUT is 0V at the time spot t0 and gradually rises up to Vc. Moreover, the rising slope of the output voltage at the output terminal OUT is also referred as a slew rate.
FIG. 2 is a schematic circuit diagram illustrating an output stage of an operational amplifier. Generally, the last stage of the operational amplifier OP is an output stage, which is a Class-AB output stage. As shown in FIG. 2, the output stage comprises a P-type transistor MPo and an N-type transistor MNo. The drain terminal of the P-type transistor MPo and the drain terminal of the N-type transistor MNo are collectively connected with the output terminal of the operational amplifier OP. Basically, the output stage may output a driving current. The driving current may be a push current or a pull current. The ways of outputting the push current or a pull current will be illustrated as follows.
Take a unity gain buffer for example. If the input voltage at the input terminal IN is higher than the output voltage at the output terminal OUT, the P-type transistor MPo of the output stage is turned on, but the N-type transistor MNo of the output stage is turned off. Under this circumstance, a push current Ipsh_op flows from the P-type transistor MPo to the output terminal OUT. Whereas, if the input voltage at the input terminal IN is lower than the output voltage at the output terminal OUT, the P-type transistor MPo of the output stage is turned off, but the N-type transistor MNo of the output stage is turned on. Under this circumstance, a pull current Ipll_op flows from the output terminal OUT to the N-type transistor MNo. That is, when the P-type transistor MPo is turned on, only the push current Ipsh_op is generated, but the pull current Ipll_op is not generated. Whereas, when the N-type transistor MNo is turned on, only the pull current Ipll_op is generated, but the push current Ipsh_op is not generated.
Since the output voltage of the operational amplifier OP has limited driving capability, the slew rate is about several volts per microsecond (V/μs). Therefore, there is a need of providing a unity-gain buffer with an enhanced slew rate.